Motion control refers to electromechanical systems which produce a desired motion in a mechanical load in response to a planned motion path. Such servo systems improve performance and productivity in automatic machinery used by manufacturing, testing, vibration control and other industries. In order to provide accurate motion control for these systems, it is necessary to accurately measure the position of the load.
FIG. 1 shows a typical prior art servo system 10 consisting of a motor 12, its moving load 14, a position sensor 16, including a stationary sensor head having two analog sensors thereon, a moving motor shaft 18 and an interpolator 20 which provides a digital signal and a servo controller 22 which uses an external digital signal and the digital signal from the interpolator 20 to provide a control signal to the motor 12 to achieve controlled linear motion. This invention can also be used to control rotary motion. The moving motor shaft 18 provides a field related to the magnet pole pair spacing thereof to be sensed, which varies along the length thereof. A measurement of the field detected by the sensors 16 can be correlated with the position of the motor shaft 18 relative to the stationary position of the motor. The variation can be sinusoidal along the length thereof or can vary in some other manner.
In one prior art system, two sensors 24 on the sensor head are located to detect a sinusoidally varying moving shaft field 25, the respective positions of the two sensor heads 24 being separated by a distance equal to 90 degrees of the sinusoidal signal period so that there is a sine signal and a cosine signal. Since the moving load 14 is driven by the moving motor shaft 18 the positions thereof are in a fixed relationship. The interpolator 20 converts easily detectable relatively coarse positioning data from the two sensors 24. The analog portions of the data from the two sensors are converted into a higher resolution signal for use by the motion control system 10 by manipulation thereof.
Typically the control signal is supplied to the system as electrical, but it can also take the form of pneumatic, hydraulic, or other power sources. The sensors can be of a type appropriate for the field being sensed, whether HALL or GMR devices for sensing magnetic flux density, or optical devices for sensing light, or other appropriate means of sensing a periodically varying field with respect to position.
It is well-known to use an arrangement of two position sensor elements as in FIG. 1 to directly output sensed signals in one of two commonly used incremental formats pictured in FIGS. 2A and 2B as for two periodic sensor signals, Analog A Quad B format 30 or in Digital A Quad B format 32.
The term incremental means that the position signal is provided in the form of electrical signals which can be used to provide a positional count by increments. The encoder signal supplied as two sine signals offset by 90 degrees (to comprise a sine/cosine pair) is commonly referred to as being in the Sin/Cos or Analog A Quad B format. The encoder signal supplied as two digital square wave signals, likewise offset in phase by 90 degrees is commonly referred to as being in the Digital A Quad B format.
If a position sensor array provides signals in either the Analog A Quad B format or in the Digital A Quad B format, a digital counter can keep track of a position measurement by counting the number of zero crossings of the sine and cosine signals over time. Since only four zero crossings are provided per electrical cycle, the zero crossing count resolution is coarse. As an example, this would provide a position measurement resolution of one quarter inch for those systems with a magnet pole pair spacing of one inch.
A common position resolution requirement in motion control is less than 1/2000th of an inch, so it is desirable to achieve a much finer position resolution than that directly output by sensing the zero crossings of the pair of sensed analog field signals. Finer resolution is provided by an interpolator.
A prior art position interpolator, such as the interpolator 20 of FIG. 1, senses a sensor signal pair in the Analog A Quad B format 34 measured at a relatively coarse resolution and converts it into a Digital A Quad B format 36 at a higher equivalent resolution as described below and as illustrated in FIGS. 3A and 3B.
The motion control system 10 can receive this Digital A Quad B formatted signal 36 which is compared to signals representing the desired path 38 to provide servo positioning signals to drive the servo motor 12 with a greater degree of precision than possible without the interpolator 20. A Digital A Quad B formatted signal can also be used by the motor drive as commutation data for certain types of motors to generate drive signals.